1. Field of the Invention
The present invention relates to a plasma CVD (Chemical Vapor Deposition) device used to deposit thin films of hydrogenated amorphous silicon and the like on a substrate.
2. Description of the Prior Art
Generally, hydrogenated amorphous silicon (hereinafter, expressed as a-Si:H) in which silicon atoms and hydrogen have been bonded together is deposited on a substrate of a glass plate or the like laminated with a metal or other electrical conductors or transparent electrically conductive films, for applications to solar cells, photoconductive drums, various sensors, thin film transistors, and the like. As the method of depositing a-Si:H onto the substrate, available are the plasma CVD process using glow discharge, the reactive sputtering process, and the like. Among others, the plasma CVD process is widely adopted as the process for fabricating a-Si:H films by virtue of its capability of depositing a-Si:H thin films having excellent film characteristics.
The plasma CVD device used for carrying out the plasma CVD process has an electrode for holding the substrate and a high frequency application electrode arranged opposite to and parallel to the electrode. In the plasma CVD device, when high frequency power is applied between the two electrodes, glow discharge is generated. A material gas is previously introduced between the two electrodes. The introduced material gas is decomposed by the glow discharge, so that an a-Si:H film is deposited on 10 the substrate held by the electrode.
In such a plasma CVD device, generally, an a-Si:H film is deposited with RF (radio frequency) power set as low as 0.05 W/cm.sup.2 so that the film forming rate will be around 0.5 to 2 .ANG./sec under the conditions of a 0.1-0.3 Torr film forming pressure and an around 200.degree. C. substrate temperature. The a-Si:H film fabricated in this way, when not doped, shows good characteristic values including an optical band gap of around 1.7 eV, a photoconductivity of around 1.times.10.sup.-4 S/cm, and a dark conductivity of around 1.times.10.sup.-10 S/cm.
When an a-Si:H film is deposited on the substrate by the plasma CVD device, it is practically preferable to fabricate the film at the fastest possible rate under such conditions that a photoconductivity of 1.times.10.sup.-5 S/cm or more, at which fabricated a-Si:H films show less deterioration of characteristics, can be obtained. That is, the faster the film forming rate is, the more the fabrication time of a-Si:H films can be reduced so that the production efficiency is improved. As a result, the fabrication cost of the a-Si:H films can be reduced. For this purpose, the film forming rate is desirably 2 .ANG./sec or more, preferably 10 .ANG./sec or more.
For example, a solar cell of the p-i-n structure, which is fabricated by depositing a-Si:H films on a substrate, has a large thickness of an i layer as much as 5000 .ANG. relative to 100-300 .ANG. thick p and n layers. Accordingly, the film forming rate of the i layer is desirably 10 or more times faster than the film forming rate of the p and n layers.
To increase the film forming rate by the plasma CVD device, it is necessary to set a high frequency power supply to high power, as well as to heighten the flow rate of material gas within a deposition chamber. However, if the material gas is fed at such a high flow rate while the high frequency power supply is set to high power, polysilane powder would be generated in plasma at low-temperature part in the deposition chamber. This leads to a problem that the generated polysilane powder would be deposited on peripheral portions of the high frequency application electrode and the side and bottom faces of the deposition chamber surrounding the high frequency application electrode. Since the density of reaction gas is relatively high at a low-temperature part and its vicinities in the deposition chamber, the material gas is fed at high flow rate such that with a high-power high frequency power applied, SiH.sub.2 radicals that have been generated in the reaction gas will react to be polymerized with SiH.sub.4 molecules. The polymerization reaction proceeds in a chain-reaction fashion, causing the generation of powder. The generated powder will be deposited on the low-temperature part and its proximate peripheries of the high frequency application electrode, and on the side and bottom faces of the deposition chamber surrounding the high frequency application electrode.
If the polysilane powder is,deposited in a large amount, the a-Si:H film will no longer be deposited uniformly while the ratio of Si-H.sub.2 bond in the resulting a-Si:H film increases because high-order silane radicals are involved in the film growth. This would incur a decrease in the photoconductivity, an increase in defect density, and the like, making it difficult to obtain a target high-quality film.
The polysilane powder, if sucked into an exhaust system, may cause the pump to decrease in its exhaust ability or to come into some malfunction. Therefore, when powder is deposited in the deposition chamber, the powder must be cleaned and removed. For removal of the powder, the film formation work must be interrupted so that the deposition chamber is turned back to normal temperature, and moreover the interior of the deposition chamber must be opened to the air. The interruption of the film formation work necessitates such processes as heating the deposition chamber and evacuating the interior of the deposition chamber for the restart of the film formation work. As a result, the availability of the plasma CVD device decreases to a considerable extent such that the advantage of high film forming rate could not be obtained.
For these and other reasons, as it stands, a-Si:H films are deposited at a film forming rate as low as 0.5-2 .ANG./sec, with a low flow rate of material gas and moreover with a high frequency low power.